Undrained Triaxial Compression Tests Laboratory Experiment ...

Undrained Triaxial Compression Tests Laboratory Experiment # 10

Date of Experiment: 4/12/2013 and 4/19/2013

CE 4718 Intermediate Soil Mechanics -Group 2 Ngoan Hoang Tommy Maude Joel Richey Sarah White

Purpose

The undrained triaxial strength tests are used to determine the shear strength of a soil sample that is not allowed to drain. The test will be completed on three unsaturated soil samples. The test results will be analyzed to determine the Mohr-Coulomb failure envelope, failure angle, shearing resistance, and Young's Modulus of Elasticity. This information can be used to predict how a soil would respond in field conditions, such as under an applied structural load or in the stability or instability of a slope.

Theory

The consolidated isotropic undrained triaxial test is the most common type of triaxial test. In this test, the saturated soil specimen is first consolidated by an all-around chamber fluid pressure, 3, which results in drainage. After the pore water pressure generated by the application of confining pressure is dissipated, the deviator stress, d, on the specimen is increased to cause shear failure. During this phase of test, the drained line from the specimen is kept closed. Because drainage is not permitted, the pore water pressure, ud, will increase. During this test, simultaneous measurements of d and ud are made. The increase in pore water pressure, ud, can be expressed in a non-dimensional form as

= ud / d where = Skempton's pore pressure parameter

The general patterns of variation of d and ud with axial strain for sand and clay soil are shown in figure below. In loose sand and normally consolidated clay, the pore water pressure increases with strain. In dense sand and overconsolidated clay, the pore water pressure increase with strain to a certain limit, beyond which it decreases and become negative (with respect to the atmospheric pressure). This decrease is because of a tendency of the soil to dilate.

Unlike the consolidated-drained test, the total and effective principal stresses are not the same in the

consolidated-undrained test. Because the pore water pressure at failure is measured in this test, the

principle stresses may be analyzed as follows:

Major principal stress at failure (total):

3 + (d)f = 1

Major principal stress at failure (effective) : 1 - (u)f = '1

Minor principal stress at failure (total):

3

Minor principal stress at failure (effective):

3 - (u)f = '3

In these equations, (u)f = pore water pressure at failure. The preceding derivations show that

1 - 3 = '1 - '3

Tests on several similar specimen with varying confining pressures may be conducted to determine the shear strength parameters. Figure below shows the total and effective stress Mohr's circles at failure obtained from consolidated ?undrained triaxial test in sand and normal consolidated clay. Noted that A and B are two total stress Mohr's circles obtained from two test. C and D are the effective stress Mohr's circles corresponding to total stress circles A and B, respectively. The diameters of circles A and C are the same, similarly, the diameter of circles B and D are the same.

In figure, the total stress failure envelope can be obtained by drawing a line that touches all the total stress Mohr's circles. For sand and normally consolidated clays, this will be approximately a straight line passing through the origin and may be expressed by the equation: f = tan (2.1) Where = total stress, = the angle that the total stress failure envelope makes with the normal stress axis, also known as the consolidated-undrained angle of shearing resistance Equation (2.1) is seldom used for practical considerations. For sand and normally consolidated clay, we can write:

= sin ?(

' = sin ?(

= sin ?(

= sin ?(

)

Again referring to the Figure, we see that the failure envelope that is tangent to all the effective stress Mohr's circles can be represented by the equation f = ' tan ', which is the same as that obtained from consolidated-drained tests.

In consolidated clays, the total stress failure envelope obtained from consolidated-undrained tests will take the shape shown in figure. The straight line a'b' is represented by the equation f = c+ tan and the straight line b'c' follows the relationship given by f = tan. The effective stress failure envelope drawn from the effective stress Mohr's circle will be similar to that so in figure.

Consolidated ?drained test on clay soils take considerable time. For this reason, consolidated-undrained test can be conducted on such soils with pore pressure measurement to obtain the drained shear strength parameters. Because drainage is not allowed in these tests during the application of deviator stress, they can be performed quickly.

Skempton's pore water pressure parameter was defined in Eq: = ud / d at failure, the parameter can be written as

= f = (ud)f / (d)f

The general range of f values in most clay soils is as follows: Normally consolidated clays: 0.5 to 1 Overconsolidated clays: -0.5 to 0

Soil Sample Description

Ottawa Sand

Material and Equipment Needed

Right-circular cylindrical specimen of cohesive soil; load frame; pressure system and water source; triaxial cell; 2 0-rings; latex membrane; membrane stretcher; vacuum grease; deformation indicator graduated to 0.001 in.; load cell or proving ring; scale with precision of0.01 g; calipers; oven-safe moisture content container; and soil drying oven set at 110? ? 5? C

Procedure

1. Obtain a soil specimen from your instructor. Use calipers to measure the initial length (La) of the specimen. Measure the diameter near the top, middle, and bottom of the specimen, and calculate the average diameter (D0) and average initial area (A0).Also measure the moist mass of the specimen (M).

2. Apply a light coating of vacuum grease to the perimeter of the base and cap to help create a waterproof seal.

3. Place the soil specimen on the base, and place the cap on top of the specimen. Make sure that the piston hole in the cap faces up.

4. Place the membrane and two O-rings on the membrane stretcher, and apply light vacuum to the membrane stretcher tube to pull the membrane towards the inside wall of the membrane stretcher.

5. The following steps describe how to place the membrane on the soil specimen:

a. Carefully lower the stretched membrane over the specimen without touching the specimen.

b. Center the membrane on the specimen and release the vacuum to allow the membrane to constrict around the specimen.

c. Gently pull the ends of the membrane over the base and cap so that the membrane surrounds the base, specimen, and cap without wrinkles.

d. With the membrane stretcher still around the specimen, carefully roll the O-rings onto the membrane where the membrane contacts the base and cap. If the base and cap are machined with grooves, make sure that the O-rings are seated in the grooves.

6. The following steps describe how to assemble the triaxial cell:

a. Place a light coating of vacuum grease on the O-rings in the pedestal and top. b. Place the cell wall on the pedestal, and make sure the pedestal and cell wall are properly

seated against one another. c. Place the top on the cell wall, and make sure the cell wall and top are properly seated

against one another. d. Slide the piston down into the hole in the cap. The tip of the piston should be far enough

into the hole to prevent the specimen from tipping when the triaxial cell is moved, but should not be applying any load to the cap. Once in position, lock the piston in place by turning the locking screw in the top. e. Tighten each of the three cell bars a little bit at a time, alternating between bars to assure an intimate seal between the pedestal, cell wall, and top.

7. Open the vent valve in the top of the triaxial cell, and begin filling the triaxial cell with water from the pedestal valve. Shut off all valves to the triaxial cell when water emerges from the vent valve.

8. Position the triaxial cell in the load frame with the deformation indicator and load cell.

9. Apply the desired cell pressure 3 to the cell through the bottom valve. You will know the specimen is under pressure when the membrane appears to be in intimate contact with the specimen.

10. Release the piston by loosening the locking screw in the top of the triaxial cell, and zero the load cell. If a proving ring is used instead of a load cell, zero the dial gauge and record the proving ring constant Kp.

11. Zero the deformation indicator. If an analog dial gauge is used, record the dial gauge conversion factor KL.

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